Defect inspection apparatus and defect inspection method

ABSTRACT

A defect inspection apparatus enabling more reliable and quicker detection of a defect present in the surface of a stacked film formed on a wafer and enabling reliable and quicker detection of a minute defect even if there is unevenness in the surface of the wafer, including a light source, a light frequency shifter unit for converting light from the light source to a plurality of beams of inspection light and a beam of reference light having close frequencies, an object lens upon which the beams of inspection light are incident and focusing the beams of light on the wafer to form a plurality of different focal points corresponding to the beams of inspection light, a laser scanning unit for making the beams of inspection light scan the wafer, a light detection unit and cofocal pinhole plate  13  for detecting an intensity of a superposed light of the beams of reflected light and the beam of reference light at a cofocal point, and an analyzing unit serving as a contrast waveform generating means for generating and combining contrast waveforms in the scanning direction at focal positions based on the light intensity detected by the optical detection unit and defect inspection method.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a defect inspection apparatus and adefect inspection method suitable for inspection of scratches, dust, andother defects present in a pattern formed in films in a process ofmanufacture of a semiconductor device having for example a stacked filmstructure.

2. Description of the Related Art

Up to the present, inspection of scratches present in a pattern on asemiconductor wafer, dust adhering to the pattern, and other defects inthe process of manufacture of a semiconductor device has been carriedout by taking the image of the semiconductor wafer creating a contrastwaveform from the two dimensional image and detecting the same.

FIG. 5 is a view schematically showing principal parts of an example ofthe configuration of a defect inspection apparatus for inspection ofscratches present in a pattern on a semiconductor wafer, dust adheringto the pattern, and other defects.

The defect inspection apparatus shown in FIG. 5 is configured so as toemit a beam of visible light from a lamp 101 via a lens 102, a halfmirror 103, and an object lens 104 onto a semiconductor wafer W and toreceive the beam of reflected light via the object lens 104, the halfmirror 103, and an image-forming lens 105 at a camera 106.

In the defect inspection apparatus having the above configuration, thebeam of light reflected from the semiconductor wafer W passes throughthe object lens 104, half mirror 103, and image-forming lens 105 to bereceived at the camera 106, a contrast waveform reflecting the surfaceshape of the semiconductor wafer W is created based on an intensity ofthe light received by the camera 106, and the defect present in apattern formed on the semiconductor wafer W is detected from thiscontrast waveform by the naked eye or the like.

FIG. 6A is a sectional view in the process of manufacture of asemiconductor device having the stacked film structure as an object tobe inspected by the defect inspection apparatus, while FIG. 6B is anexample of the contrast waveform of the surface shape of thesemiconductor device shown in FIG. 6A by the defect inspectionapparatus.

In the structure of the semiconductor device shown in FIG. 6A, forexample, a silicon oxide pattern SP is formed on the wafer W, and analuminum interconnection pattern AP is formed on this silicon oxidepattern SP.

As will be understood from FIGS. 6A and 6B, the contrast waveform at astep difference between the resist pattern RP and the aluminuminterconnection pattern AP has a different shape from the actual shapedue to the coverage of the aluminum.

Summarizing the problem to be solved by the invention, the shape of thestep difference present between the resist pattern RP and the aluminuminterconnection pattern AP cannot be accurately determined from a regionPA and a region Pr of the contrast waveform obtained by the defectinspection apparatus.

In the defect inspection apparatus, when there is a relatively deep andinclined step difference in the pattern like the inclined surfaces ofthe resist pattern RP and the aluminum interconnection pattern AP, thereis a disadvantage that even if there is for example a scratch, adhesionof dust, or another defect in these inclined surfaces, it cannot bedetected.

In the related art, when there is a relatively deep and inclined step asdescribed above in the pattern stacked on the semiconductor wafer, forexample a scanning electron microscope (SEM) has been used to conduct asampling inspection from among a large number of semiconductor wafers.

In defect inspection using a scanning electron microscope, however, thenumber of the scanning electron microscopes which can be introduced islimited from the viewpoint of the plant and apparatus investment sincescanning electron microscopes are high in cost. Further, the number ofthe semiconductor wafers inspected is limited since the throughput ofthe inspection is low.

Semiconductor wafers not inspected for defects are sent to the nextstep. These uninspected semiconductor wafers have become a cause ofreduction of the yield of the product.

In the future, in the process of manufacture of a semiconductor device,along with the miniaturization of the circuit pattern of integratedcircuits, employment of a stacked film structure of more layers for thesemiconductor device cannot be avoided, so development of a defectinspection apparatus which can correctly inspect minute defects existingin a semiconductor wafer in the process of manufacture of asemiconductor device at a low cost and with a high throughput has becomenecessary.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a defect inspectionapparatus and a defect inspection method capable of more reliably andquickly detecting defects existing in patterns formed in the stackedfilms formed on a wafer in for example a process of manufacture of asemiconductor device, more particularly capable of reliably and quicklydetecting minute defects even if there are step differences and otherunevenness at the surface of the wafer.

According to a first aspect of the present invention, there is provide adefect inspection apparatus for inspecting for a defect existing in ainspected surface, comprising a light source for emitting a beam oflight of a predetermined frequency band, a light frequency changingmeans for receiving as its input the is beam of light emitted from thelight source and outputting the related beam of light converted to aplurality of beams of inspection light having close frequenciesdifferent from each other and a beam of reference light beam, a lightfocusing means upon which the beams of inspection light output from thelight frequency changing means are incident through an identical opticalpath and focusing the related beams of inspection light to the inspectedsurface to forming a plurality of different focal points correspondingto the beams of inspection light, a scanning means for scanning thefocused beams of inspection light on the inspected surface, asuperposing means for superposing the beam of the reflected light of theinspection light from the inspected surface and the beam of referencelight to cause interference between the beams of reflected light and thebeam of reference light, a light receiving means upon which thesuperposed light of the beams of reflected light and the beam ofreference light is incident and detecting the intensity of thesuperposed light by confocal detection, and a contrast waveformgenerating means for generating contrast waveforms in the scanningdirection at the focal positions in respect to the light intensitydetected by the light receiving means and combining the contrastwaveforms.

Preferably, the apparatus further comprises a defect detecting means fordetecting defects of the pattern based on the contrast waveforms.

Preferably, the light receiving means has a light receiving element forreceiving the superposed light and a pin hole plate provided in anincident light path of the superposed light to the light receivingelement and having a small aperture and detecting the intensity of thesuperposed light by confocal detection.

Preferably, the light source selectively output a beam of visible laserlight of a visible band and a beam of far-ultraviolet laser light of afar-ultraviolet band.

Preferably, the superposing means has beam splitters for reflecting thebeam of reference light output from the light frequency changing meansto the light receiving means, passing the beams of reflected light fromthe inspected surface striking it after following an identical opticalpath as that for the beams of inspection light, and exposing the same tothe light receiving means.

Preferably, the light frequency shifting means has a plurality ofacousto-optic modulating means for changing the frequency of the lightemitted from the light source by supersonic waves of differentfrequencies from each other.

Preferably, the inspected surface comprises uneven surface.

Preferably, the inspected surface comprises the surface of a filmstacked on a semiconductor substrate and formed into a predeterminedpattern.

More preferably, the patterns are formed symmetrical about apredetermined center line, and the defect detecting means detects a partwhich is not symmetrical about the center line in the contrast waveformdata obtained by the contrast waveform generating means as a defect.

Preferably, the scanning means has a galvanomirror or a supersonic lightpolarization element for scanning the beams of inspection light on theinspected surface.

Preferably, the scanning means two-dimensionally scans the beams ofinspection light on the inspected surface, and the contrast waveformgenerating means generates a contrast image reflecting athree-dimensional shape of the inspected surface from the combinedcontrast waveforms obtained as a result of the scannings.

According to a second aspect of the present invention, there is provideda defect inspection apparatus for inspecting for a defect present in aninspected surface, comprising a light source for emitting a beam oflight of a predetermined frequency band, a light frequency changingmeans for receiving as its input the beam of light emitted from thelight source and outputting the related beam of light converted to aplurality of beams of inspection light and a beam of reference lightbeam having close frequencies different from each other, a lightfocusing means upon which the beams of inspection light output from thelight frequency changing means are incident through an identical opticalpath and focusing the related beams of inspection light to the inspectedsurface to form a plurality of different focal points corresponding tothe beams of inspection light, a scanning means for making the focusedbeams of inspection light scan the inspected surface, a superposingmeans for superposing beams of reflected light of the beams ofinspection light from the inspected surface and the beam of referencelight on each other to cause interference between the related beams ofreflected light and the beam of reference light, a light receiving meansupon which the superposed light of the beams of reflected light and thebeam of reference light is incident and detecting the intensity of thesuperposed light, a contrast waveform generating means for generatingcontrast waveforms in the scanning direction at the focal positionsbased on the light intensity detected by the light receiving means andcombining the contrast waveforms, and a defect detecting means fordetecting a defect of a pattern based on the contrast waveforms.

Preferably, the inspected surface comprises the surface of a filmstacked on a semiconductor substrate and formed into a predeterminedpattern.

Preferably, the patterns are formed symmetrical about a predeterminedcenter line and the defect detecting means detects a part which is notsymmetrical about the center line in the contrast waveform data obtainedby the contrast waveform generating means as a defect.

According to a third aspect of the present invention, there is provideda defect inspection apparatus for inspecting for a defect present in aninspected surface, comprising a light source for emitting a beam oflight of a predetermined frequency band, a light frequency changingmeans for receiving as its input the beam of light emitted from thelight source and outputting the related beam of light converted to aplurality of beams of inspection light and a beam of reference lightbeam having close frequencies different from each other, a lightfocusing means for focusing the related beams of inspection light outputfrom the light frequency changing means on the inspected surface to formfocal points, a scanning means for scanning the focused beams ofinspection light on the inspected surface, a superposing means forsuperposing the beam of reflected light of the beams of inspection lightfrom the inspected surface and the beam of reference light on each otherto cause interference between the beams of reflected light and the beamof reference light, a light receiving means upon which the superposedlight of the beams of reflected light and the beam of reference light isincident and detecting the intensity of the superposed light by confocaldetection, and a contrast waveform generating means for generatingcontrast waveforms in the scanning direction at the focal positionsbased on the light intensity detected by the light receiving means.

Preferably, the apparatus further comprises a defect detecting means fordetecting defects of the pattern based on the contrast waveforms.

Preferably, the inspected surface comprises the surface of a filmstacked on a semiconductor substrate and formed into a predeterminedpattern.

More preferably, the patterns are formed symmetrical about apredetermined center line, and the defect detecting means detects a partwhich is not symmetrical about the center line in the contrast waveformdata obtained by the contrast waveform generating means as a defect.

Preferably, the light receiving means has an aperture plate having asmall aperture for detecting the intensity of the superposed light byconfocal detection in the incident optical path of the superposed light.

According to a fourth aspect of the present invention, there is provideda defect inspection method for inspecting for a defect present in aninspected surface, comprising converting light of a predeterminedfrequency band to a plurality of beams of inspection light and a beam ofreference light having close frequencies different from each other,passing the plurality of beams of inspection light through the identicaloptical path and focusing them on the inspected surface to form aplurality of different focal points corresponding to the beams of theinspection light and scanning them the inspected surface, superposingthe beams of reflected light of the beams of inspection light from theinspected surface and the beam of reference light on each other to causeinterference between them and detecting the intensity of the relatedsuperposed light at the confocal point, generating contrast waveforms inthe scanning direction at the focal positions based on the detectedlight intensity, combining the contrast waveforms, and detecting adefect of the inspected surface based on the combined contrast waveform.

Preferably, the method in the light frequency changing step, a beam offar-ultraviolet laser light of the far-ultraviolet band is used as thebeam of light of the predetermined frequency band.

Preferably, at least part of the inspected surface comprises the surfaceformed.

Preferably, the method further comprises a step of selecting and usinglight of a frequency band differing according to the type of the objectto be inspected constituting the inspected surface for the light of thepredetermined frequency band.

Alternatively, preferably, the method further comprises a step ofselecting and using one of a beam of far-ultraviolet laser light of thefar-ultraviolet band and a beam of visible laser light of the visibleband for the light of the predetermined frequency band according to thetype of the object to be inspected constituting the inspected surface.

More preferably, the method further comprises a step of using the beamof visible laser light for inspection of an inspected surfaceconstituted by a material such as polycrystalline silicon having arelatively low spectral reflectance for light of a short wavelength anda step of using the beam of far-ultraviolet laser light for inspectionof an inspected surface formed by a material having a relatively highspectral reflectance for light of a short wavelength.

Preferably, the inspected surface comprises the surface of a filmstacked on a semiconductor substrate and formed into a predeterminedpattern.

More preferably, the patterns are formed symmetrical about apredetermined center line, and a part which is not symmetrical about thecenter line in the combined contrast waveform data is detected as adefect.

Preferably, the method further comprises two-dimensionally making thebeams of inspection light scan the inspected surface, generating acontrast image reflecting a three-dimensional shape of the inspectedsurface from the combined contrast waveforms obtained as a result of thescannings, and detecting a defect of the inspected surface based on thecontrast image.

Preferably, the method further comprises two-dimensionally making thebeams of inspection light scan the inspected surface, generating acontrast image reflecting a three-dimensional shape of the inspectedsurface from the combined contrast waveforms obtained as a result of thescannings, and detecting a defect of the inspected surface based on thecontrast image.

According to a fifth aspect of the present invention, there is provideda defect inspection method for inspecting for a defect present in aninspected surface, comprising converting light of a predeterminedfrequency band to a beam of inspection light and a beam of referencelight having close frequencies different from each other, focusing thebeam of inspection light on the inspected surface to form a focal pointand scanning it the inspected surface, superposing the beam of reflectedlight of the beam of inspection light from the inspected surface and thebeam of reference light on each other to cause interference between themand detecting the intensity of the related superposed light by confocaldetection, generating a contrast waveform in the scanning direction atthe focal position based on the detected light intensity, and detectinga defect of the inspected surface based on the contrast waveform.

That is, in the present invention, light emitted from a light source isconverted to a plurality of beams of inspection light and a beam ofreference light having close frequencies different from each other bythe light frequency changing means.

The plurality of beams of inspection light output from the lightfrequency changing means are focused on the inspected surface by thefocusing means through the identical optical path. The focused beams ofinspection light have frequencies different from each other, so aplurality of different focal points are formed with respect to theinspected surface. The focused beams of inspection light are made toscan the inspected surface by the scanning means, the beams of reflectedlight from the inspected surface are superposed on the beam of referencelight by the superposing means, and a beat of the differential frequencyis produced by the superimposition.

The beat of the differential frequency is detected by the lightreceiving means. The detection of the beat of the differential frequencygenerated by interference between these beams of inspection light andthe beam of reference light is referred to as optical heterodynedetection. By the optical heterodyne detection, the contrastcharacteristic of the image to be obtained is improved, and an S/N ratioof the detection light is improved.

Further, when detecting the optical intensity by optical heterodynedetection, the light receiving means detects the intensity by confocaldetection.

Confocal detection is a detection method for making the reflected lightpass through a pinhole, slit, or other aperture and strike the lightreceiving surface of the light receiving means and detecting theintensity of part of a range including the center portion of thedistribution of the intensity of the reflected light. The opticalresolution is improved by the confocal detection, and the contrastcharacteristic of the obtained image is improved.

The intensity information of the reflected light obtained at the lightreceiving means by the optical heterodyne detection and the confocaldetection includes intensity information of the reflected light obtainedat a plurality of different focal positions, so the contrast waveformgenerating means generates contrast waveforms in the scanning directionat the focal positions and combines the generated contrast waveforms.

The combined contrast waveform is a combination of contrast waveformsobtained at a plurality of different focal positions, therefore even ifthere is a certain degree of unevenness in a depth direction in thesurface shape of the inspected surface, a shape faithfully reflectingthe surface shape of the inspected surface is exhibited.

A defect present in the inspected surface can be detected by thedifference of the surface shape of the specified inspected surface froman intended shape based on the combined contrast waveform.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and features of the present invention will be moreapparent from the following description of the preferred embodimentswith reference to the accompanying drawings, wherein:

FIG. 1 is a view of the configuration of a defect inspection apparatusaccording to an embodiment of the present invention;

FIG. 2 is a view of the state where a plurality of beams of inspectionlight are focused by the object lens to form a plurality of differentfocal points;

FIG. 3 is a view of the sectional structure of a pattern stacked on awafer and an example of the contrast waveform generated in the analyzingunit 21;

FIG. 4A is a view of an example of the combined contrast waveform whenthere is a defect in the pattern, and

FIG. 4B is a view of the state when the combined contrast waveform isfolded at the center line O and superposed;

FIG. 5 is a view schematically showing the principal parts of theconfiguration of a defect inspection apparatus of a two-dimensionalimage for inspecting for a defect of patterns formed on the wafer; and

FIG. 6A is a sectional view of the process for manufacture of asemiconductor device having a stacked film structure, and

FIG. 6B is a view of the contrast waveform thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, an explanation will be made of a preferred embodiment of thepresent invention by referring to the drawings.

FIG. 1 is a view of the configuration of an embodiment of the defectinspection apparatus of the present invention.

A defect inspection apparatus 1 shown in FIG. 1 has a laser scanningtype confocal microscope portion provided with a light source 2, a lightfrequency shifter unit 6 for shifting the frequency of the laser light,a laser scanning unit 11 for making the beam of laser light scan, anoptical detection unit 12, a confocal pinhole plate 13, a camera unit22, an image-forming lens 23, and an object lens 24 and a stage whichholds an analyzing unit 21, the camera unit 22, the image-forming lens23, and a wafer W as the object to be inspected and can control thepositioning of the wafer W in an X-direction, Y-direction, and aZ-direction with a high precision.

The light source 2 has a far-ultraviolet laser light source 3 and avisible laser light source 4.

The far-ultraviolet laser light source 3 outputs the far-ultravioletlaser light of the far-ultraviolet band having a relatively shortwavelength.

The visible laser light source 4 outputs the visible laser light of thevisible band.

The laser light output from the far-ultraviolet laser light source 3passes through a beam splitter BS2 provided in the light source 2 and isoutput to a predetermined optical path outside the light source 2.

The laser light output from the visible laser light source 4 isreflected at a beam splitter BS1 provided in the light source 2 tostrike the beam splitter BS2, reflected at the beam splitter BS2, andoutput to a predetermined optical path outside the light source 2.

The light source 2 has a mechanism capable of selecting and emitting thevisible laser light of the visible band and the far-ultraviolet laserlight of the far-ultraviolet band having a relatively short wavelengthaccording to the type, purpose of the inspection, etc. of the object tobe inspected.

In the present embodiment, with laser light of a short wavelength of forexample 400 nm or less, visible laser light having a relatively longwavelength is used for the inspection for a defect present in thepattern formed by a material having a low spectral reflectance, forexample, polycrystalline silicon.

Even with laser light of a short wavelength of for example 400 nm orless in which the problem as described above does not occur,far-ultraviolet laser light having a high optical resolution and shortwavelength is used for the inspection for a defect of an interconnectionpattern formed by a material such as aluminum having a relatively highspectral reflectance.

The light frequency shifter unit 6 has acousto-optic modulators (AOM)AOM0, AOM1, AOM2, and AOM3 and beam splitters BS4 to BS7 which eachsplit one of the beams of laser light output from the light source 2 andsplit into two by the beam splitter BS3 and make it strike upon theacousto-optic modulators AOM0, AOM1, AOM2, and AOM3.

The acousto-optic modulators AOM0, AOM1, AOM2, and AOM3 add supersonicwaves of different frequencies to the incident beams of laser light L toshift the frequencies to close frequencies different from each other andoutput beams of laser light having frequencies different from eachother.

The acousto-optic modulator AOM0 shifts the frequency of the beam oflaser light L to convert it to the beam of reference light L0 andoutputs this, but it is also possible to adopt a configuration whichdoes not change the frequency of the beam of laser light L, passes it asit is, and makes the frequency of the reference light L0 the samefrequency as that of the beam of laser light L.

The acousto-optic modulators AOM1, AOM2, and AOM3 shift the beam oflaser light L in frequency to output the beams of inspection light L1,L2, and L3.

On the output side of the light frequency shifter unit 6, beam splittersBS8 to BS11 are provided corresponding to the acousto-optic modulatorsAOM0, AOM1, AOM2, and AOM3.

The beam splitter BS8 reflects the beam of reference light L0 outputfrom the acousto-optic modulator AOM0 and makes it strike a confocalpinhole 13 a of the confocal pinhole plate 13.

The beam splitters BS9 to BS11 reflect the beams of inspection light L1,L2, and L3 output from the acousto-optic modulators AOM1, AOM2, and AOM3and make them strike the laser scanning unit 3 through the identicaloptical path.

Note that the beam splitters BS8 to BS11 pass the beams of reflectedlight output from the laser scanning unit 11 therethrough and make themstrike the confocal pinhole 13 a of the confocal pinhole plate 13.

The laser scanning unit 3 is exposed the beams of inspection light L1,L2, and L3 from the light frequency shifter unit 6 which are reflectedat the beam splitters BS9 to BS11 and pass through the identical opticalpath and scans and outputs these beams of inspection light L1, L2, andL3.

The laser scanning unit 11 can be configured by for example agalvanomirror or supersonic light polarization element though themechanism is not illustrated here.

The object lens 24 is provided so that its optical axis becomesperpendicular relative to the stage 25 and focuses the beams ofinspection light L1, L2, and L3 output from the laser scanning unit 11reflected by the beam splitter BS13 provided above the object lens 24 toform the focal points on the wafer W held on the stage 25.

The beams of inspection light L1, L2, and L3 output from theacousto-optic modulators AOM1, AOM2, and AOM3 have frequencies differentfrom each other, therefore, for example, as shown in FIG. 2, the focalpoints of the beams of inspection light L1, L2, and L3 focused by theobject lens 24 are formed at different focal positions f1, f2, and f3 inthe Z-direction, that is, a height direction of the wafer W.

The optical detector 12 measures the intensity of the light obtained bysuperposing the beams of reflected light of the beams of inspectionlight L1, L2, and L3 striking the wafer W and the beam of referencelight L0, converts this to an electric signal, and outputs the same tothe analyzing unit 21.

The beams of reflected light of the beams of inspection light L1, L2,and L3 striking the wafer W follow the optical path of the beams ofinspection light L1, L2, and L3 and strike the optical detector 12.

Namely, the beams of reflected light pass through the beam splittersBS11 to BS8 via the object lens 24, beam splitter BS13, and the laserscanning unit 11 to strike the optical detector 12. At this time, thebeam of reference light L0 output from the acousto-optic modulator AOM0also passes through the identical optical path as that for the beams ofreflected light and strikes the optical detector 12, so the beam ofreference light L0 and the beams of reflected light are superposed.

The optical detector 12 can be constituted by for example a photodiode.

The confocal pinhole plate 13 is formed with a confocal pinhole 13 a asan aperture for confocal detection of the intensity of the superposedlight by the optical detector 12 and is provided at a predeterminedposition relative to the optical detector 12.

The analyzing unit 21 generates the contrast waveforms along thescanning direction of the laser scanning unit 11 at focal positions f1,f2, and f3 on the wafer W based on the detection signal of the opticaldetector 12.

Further, the analyzing unit 21 combines the generated contrast waveformsat the focal positions f1, f2, and f3.

Further, the analyzing unit 21 detects a scratchy, dust, or other defecton the wafer W based on the combined contrast waveform. Note that thespecific detection method of the analyzing unit 21 will be explainedlater.

The camera unit 22 is provided for observation of an illumination imageobtained by the beam of laser light, in the laser light L emitted fromthe light source 2, which is split at the beam splitter BS3 and strikesthe wafer W via the beam splitters BS12 and BS13 and the object lens 24and an illumination image of a lamp etc.

The image-forming lens 22 is a lens for forming an image of the wafer Won the camera unit 22.

In the defect inspection apparatus 1 having the above configuration, forexample, a wafer W having a stacked film structure is mounted on thestage 25. The wafer W is scanned by the beams of inspection light L1,L2, and L3 shifted in their frequencies at the light frequency shifterunit 6 by the laser scanning unit 11 and striking the wafer W via theobject lens 24.

The beams of reflected light reflected at the wafer W pass through theconfocal pinhole 13 a of the confocal pinhole plate 13 together with thebeam of reference light L0 shifted in its frequency at the lightfrequency shifter unit 6 via the object lens 24 and are received at theoptical detection unit 12.

Namely, the defect inspection apparatus 1 performs optical heterodynedetection where the beams of reflected light reflected from the wafer Ware superposed on the beam of reference light L0 and the beat of thesuperposed light (phenomenon where the intensity of the light changes bytime by the differential frequency) at the optical detection unit 12.

Simultaneously, the superposed light passes through the confocal pinhole13 a for confocal detection.

Optical heterodyne detection is a method of detection of causinginterference between two beams of laser light having differentfrequencies from each other, receiving this by the optical detector,generating the beat of the differential frequency produced by thesuperimposition of light waves having close frequencies, and measuring adistance from a phase change of this beat.

By using optical heterodyne detection, the contrast characteristic andthe S/N ratio of the detection light can be improved.

In the defect inspection apparatus 1, use of optical heterodynedetection enables inspection with a very weak light and enables use of afar-ultraviolet laser light source 3 for the light source 2.

For this reason, for example, in the case when the object to beinspected is formed by a material such as a resist having aphotosensitivity with respect to light, inspection to find defects inthe surface of the material while preventing exposure of thephotosensitive material by the very weak light becomes possible.

By using far-ultraviolet laser light having a short wavelength, theoptical resolution can be improved.

Confocal detection is a detection method for making beams of reflectedlight pass through a pinhole, slit, or other aperture to receive thelight and detecting the intensity of part of the range including thecenter portion of the distribution of intensity of the reflected light.The various types of light noise accompanying the reflected light arecut so as not to enter into the light receiving surface. Only theintensity in a so-called Airy disk is measured. Therefore the contrastcharacteristic of the image can be improved, and the optical resolutioncan be improved.

Next, an explanation will be made of an example of a method forinspection of a defect on a wafer W using a defect inspection apparatus1 having the above configuration.

Referring to FIG. 3, an explanation will be given taking as an example acase, in a process of manufacture of a semiconductor device having astacked film structure, of inspecting if there are minute defects in analuminum interconnection pattern AP and a resist pattern RP on a patternSP made of silicon oxide on a wafer W in an integrated circuit process,for example, the aluminum interconnection step.

First, the wafer W is mounted on the stage 25 of the defect inspectionapparatus 1.

A predetermined alignment of the wafer W is carried out, and the stage25 is moved to a predetermined position.

In this state, the beam of laser L is shifted in frequency to the beamsof laser having close frequencies different from each other, andconverted to the beams of incident light L0 and inspection light L1, L2,L3.

Next, the beams of inspection light L1, L2 and L3 shifted in frequencyat the acousto-optic modulators AOM1, AOM2, and AOM3 of the lightfrequency shifter unit 6 pass the object lens 24 to expose the wafer Wand form focal points at different focal positions f1, f2, and f3.

The beams of inspection light is scanned in the direction X by the laserscanning unit 11.

The beams of reflected light of the beams of the inspection lightstroked and reflected by the wafer are superposed with the beam of thereference light L0.

The superposed light of the beams is passed the confocal pin-hole 13 a,and detected the intensity of the superposed light of the beams byconfocal detection by the light detection unit 12.

Next, the contrast waveforms in the scanning direction at the focalpositions f1, f2, f3 based on the intensity of the superposed light ofthe beams detected by confocal detection by the light detecting unit 12.

FIG. 3 shows the sectional structure of a pattern stacked on the wafer Wand an example of the contrast waveform created at the analyzing unit21, in which FIG. 3A shows the contrast waveform at the focal positionf1, FIG. 3B shows the contrast waveform at the focal position f2, andFIG. 3C shows the contrast waveform at the focal position f3.

As seen from FIG. 3A to FIG. 3C, the contrast waveforms become waveformsreflecting the shapes in the vicinity of the focal positions f1, f2. andf3, but do not reflect the shapes at positions away from the focalpositions f1, f2, and f3.

The correct three-dimensional shape of the stacked pattern on the waferW cannot be specifically determined from the contrast waveforms.

In the analyzing unit 21, the contrast waveforms at the focal positionsf1, f2, and f3 are combined.

FIG. 3D shows the contrast waveform obtained by combining the contrastwaveforms at the focal positions f1, f2, and f3.

As seen from FIG. 3D, if the contrast waveforms at the focal positionsf1, f2, and f3 are combined, a contrast waveform correctly reflectingthe surface shape of the pattern stacked on the wafer W can be obtained.

As a result, it becomes possible to specifically determine the surfaceshape of the pattern stacked on the wafer W.

When there is a defect in the surface of the pattern stacked on thewafer W, for example, if there is a defect in the inclined surface ofthe step portion of for example a pattern AL, the combined contrastwaveform becomes the waveform as shown in for example FIG. 4A.

In FIG. 4A, a defect portion K is formed in the combined contrastwaveform corresponding to the defect in the inclined surface of the stepportion of the pattern AL.

By confirming the defect portion K of the contrast waveform by forexample the naked eye, the defect can be detected at the inclinedsurface of the step portion of the pattern AL.

Further, in the analyzing unit 21 of the defect inspection apparatus 1according to the present embodiment, the defect K is detected by forexample the following method.

The resist pattern RP and the aluminum pattern AP are formed linearlysymmetrically about a predetermined center line O (center of thesuperimposition) as shown in FIGS. 4A and 4B.

In the analyzing unit 21 of the defect inspection apparatus 1, thecombined contrast waveform data is folded at the center line focal O andsuperposed as shown in FIG. 4B.

There is no defect in the contrast waveform on the right side of thecenter line O, therefore a part K1 which is not symmetrical is generatedin the contrast waveform superposed about the center line O.

The analyzing unit 21 of the defect inspection apparatus 1 detects thisasymmetrical part K1 to detect the defect.

As described above, by the defect inspection apparatus 1 according tothe present embodiment, the three-dimensional shape of the resistpattern RP and the aluminum pattern AP having a large step differencewhich could not be correctly measured by the defect inspection apparatusfor a two-dimensional image of the related art can be correctlyspecified from the combined contrast waveform.

As a result, it becomes possible to detect a minute defect present inthe wafer W surface on which the resist pattern RP and the aluminumpattern AP are formed based on the combined contrast waveform.

In the defect inspection apparatus 1 according to the presentembodiment, the laser scanning unit 3 repeatedly performs the operationof making the beams of inspection light L1, L2, and L3 scan the surfaceof the wafer W in for example the X-axial direction and then moving thestage 25 by a predetermined amount in a Y-axial direction and makingthem scan again in the X-axial direction, whereby the beams ofinspection light L1, L2, and L3 can be made to scan the surface of thewafer W two-dimensionally.

The analyzing unit 21 of the defect inspection apparatus 1 can alsogenerate a contrast image reflecting the three-dimensional shape of thesurface of the entire wafer W from the combined contrast waveformobtained as a result of the scannings in the two-dimensional scanning asdescribed above.

By employing such a configuration, the defect inspection can be carriedout while viewing the entire wafer W surface, therefore inspectionbecomes easy.

Further, in the present embodiment, the optical resolution is improvedby the confocal detection, and real time three-dimensional measurementcan be carried out by optical heterodyne detection and simultaneousdetection at a plurality of focal positions.

Note that, in the above-mentioned embodiment, the light frequencyshifter used three acousto-optic modulators, but if the number of thisis increased, the precision of the contrast waveform can be improved.

For example, an acousto-optic deflector (AOD) and an acousto-opticmodulator using a surface acoustic wave (SAW) can be used for the lightfrequency shifter unit 6.

The defect inspection apparatus 1 is not limited to the case ofinspecting for a defect present in the resist pattern RP and thealuminum pattern AP in the aluminum interconnection step. It can beapplied to cases of inspecting for defects of various patterns and filmsin semiconductor devices having a variety of stacked film structures invarious steps.

According to the present embodiment, by providing the confocal pinholeplate 13 in the optical detector 12 and detecting the intensity of thereflected light by confocal detection, the optical resolution can beimproved and the contrast characteristic can be improved.

By using optical heterodyne detection superposing the beams ofinspection light and beam of reference light with different frequenciesand detecting the beat of the differential frequency generated due tothe interference between the beams of inspection light and the beam ofreference light at the optical detector 12, the precision of thecontrast waveform is improved and the S/N ratio of the light to bedetected can be improved.

According to the present embodiment, by using optical heterodynedetection, even if relatively weak light is used as the laser light, acontrast waveform with a high precision is obtained, therefore,far-ultraviolet laser light having a relatively short wavelength can beused for the laser light, the optical resolution can be furtherimproved, and even if very weak light is used, a contrast waveform witha high precision is obtained, therefore, a precise defect inspection ofa material such as a resist having photosensitivity becomes possible.

Up to the present, in order to obtained a three-dimensional shape, forexample, a plurality of two-dimensional images are acquired while movinga stage holding a wafer in the incident direction of the measurement uselaser and these images are combined so as to combine a three-dimensionalimage. Due to this configuration, a long time is required for obtainingthe three-dimensional shape and the precision of the obtained image islowered since the stage is moved, but in the present embodiment, thethree-dimensional shape can be instantaneously obtained and also theprecision of the obtained image becomes high.

In the present embodiment, three acousto-optic modulators were used inthe light frequency shifter unit 6 in order to create the beams ofinspection light, but if the number is further increased, a furtherprecise contrast waveform can be obtained and the precision of thedefect inspection can be further improved.

Note that the present invention is not limited to the above embodiment.

For example, a configuration can be employed not using the confocalpinhole plate 13 and not performing confocal detection. In this case,the contrast characteristic is slightly lowered since confocal detectionis not carried out, but the hardware configuration can be simplified.

In the above embodiment, different focal positions were formed on thewafer W by using a plurality of acousto-optic modulators AOM1 to AOM3,but it is also possible to employ for example a configuration forinspection by a single focal point without forming a plurality of focalpoints by using a single acousto-optic modulator AOM.

For example, when the surface shape of the wafer W has a relativelyshallow step portion, a contrast waveform having an improved contrastcharacteristic by confocal detection and optical heterodyne detectioncan be obtained and the surface shape can be correctly specified.

Due to the present invention, not only inspection for defects present inthe pattern of a semiconductor device of a stacked film structure, butalso for example observation and defect inspection of the shape in acontact hole having a high aspect ratio, inner surface shape, and bottomof the contact hole are possible, thus the present invention can beapplied to defect inspection of the inspected surfaces of a variety ofobjects having unevenness.

Summarizing the effects of the invention, according to the presentinvention, by using optical heterodyne detection, measurement with thevery weak light becomes possible and a far-ultraviolet laser can be usedfor the light source.

According to the present invention, by forming a plurality of differentfocal points and finding the contrast waveforms at the focal positions,real time measurement of the three-dimensional shape becomes possibleand also a shortening of the measurement time can be achieved.

According to the present invention, by repeating the inspection whilemoving a plurality of different focal points in a depth direction of thethree-dimensional object to be inspected, high precisionthree-dimensional information can be obtained and the present inventioncan be applied to also three-dimensional image observation. For example,the observation and measurement of the bottom of a contact hole having alarge aspect ratio are possible.

The defect inspection apparatus and method according to the presentinvention is effective not only to the defect inspection of the flatinspected surface but also the same of inspected surface with uneveness,differences in level or grooves.

What is claimed is:
 1. A defect inspection apparatus for inspecting fora defect in an inspected surface, comprising: a light source emitting abeam of light of a predetermined frequency band; a light frequencychanging means for receiving as its input the beam of light emitted fromthe light source and outputting the related beam of light converted to aplurality of beams of inspection light and a beam of reference lightbeam having close frequencies different from each other; a lightfocusing means upon which the beams of inspection light output from thelight frequency changing means are incident through an identical opticalpath and focusing the related beams of inspection light to the inspectedsurface to form a plurality of different focal points corresponding tothe beams of inspection light; a scanning means for scanning the focusedbeams of inspection light on the inspected surface; a superposing meansfor superposing the beam of the reflected light of the inspection lightfrom the inspected surface and the beam of reference light to causeinterference between the beams of reflected light and the beam ofreference light; a confocal defecting means upon which the superposedlight of the beams of reflected light and the beam of reference light isincident and detecting the intensity of the superposed light by confocaldetection; and a contrast waveform generating means for generatingcontrast waveforms reflecting a surface shape of the inspected surfacein the scanning direction at the focal positions in response to thelight intensity detected by the confocal defecting means and combiningthe contrast waveforms.
 2. A defect inspection apparatus as set forth inclaim 1, further comprising a defect detecting means for detectingdefects of the pattern based on the contrast waveforms.
 3. A defectinspection apparatus as set forth in claim 1, wherein said confocaldefecting means has: a pin hole aperture plate having a small aperturefor passing the superposed light and a light receiving element anddetecting the intensity of the superposed light passed said aperture. 4.A defect inspection apparatus as set forth in claim 1, wherein the lightsource selectively outputs a beam of visible laser light of a visibleband and a beam of far-ultraviolet laser light of a far-ultravioletband.
 5. A defect inspection apparatus as set forth in claim 1, whereinthe superposing means has beam splitters for reflecting the beam ofreference light output from the light frequency changing means to thecontrol defecting means, passing the beams of reflected light from theinspected surface striking it after following an identical optical pathas that for the beams of inspection light, and exposing the same to thefocal detecting means.
 6. A defect inspection apparatus as set forth inclaim 1, wherein the light frequency shifting means has a plurality ofacousto-optic modulators means for changing the frequency of the lightemitted from the light source by supersonic waves of differentfrequencies from each other.
 7. A defect inspection apparatus as setforth in claim 1, wherein the inspected surface comprises unevensurface.
 8. A defect inspection apparatus as set forth in claim 1,wherein the inspected surface comprises the surface of a film stacked ona semiconductor substrate and formed into a predetermined pattern.
 9. Adefect inspection apparatus as set forth in claim 8, wherein: thepatterns are formed symmetrical about a predetermined center line andthe defect detecting means detects a part which is not symmetrical aboutthe center line in the contrast waveform data obtained by the contrastwaveform generating means as a defect.
 10. A defect inspection apparatusas set forth in claim 1, wherein the scanning means has a galvanomirroror a supersonic light polarization element for scanning the beams ofinspection light on the inspected surface.
 11. A defect inspectionapparatus as set form in claim 1, wherein: the scamming meanstwo-dimensionally scans the beams of inspection light on the inspectedsurface and the surface shape of the inspected surface generated by thecontrast waveform generating means is a contrast image reflecting athree-dimensional shape of the inspected surface from the combinedcontrast waveforms obtained as a result of the scannings.
 12. A defectinspection apparatus for inspecting for a defect present in an inspectedsurface, comprising: a light source for emitting a beam of light of apredetermined frequency band; a light frequency changing means forreceiving as its input the beam of light emitted from the light sourceand outputting the related beam of light converted to a plurality ofbeams of inspection light and a beam of reference light beam havingclose frequencies different from each other; a light focusing means uponwhich the beams of inspection light output from the light frequencychanging means are incident through an identical optical path andfocusing the related beams of inspection light to the inspected surfaceto form a plurality of different focal points corresponding to the beamsof inspection light; a scanning means for making the focused beams ofinspection light scan the inspected surface; a superposing means forsuperposing beams of reflected light of the beams of inspection lightfrom the inspected surface and the beam of reference light on each otherto cause interference between the related beams of reflected light andthe beam of reference light; a light receiving means upon which thesuperposed light of the beams of reflected light and the beam ofreference light is incident and detecting the intensity of thesuperposed light; and a contrast waveform generating means forgenerating contrast waveforms reflecting a surface shape of theinspected surface in the scanning direction at the focal positions basedon the light intensity detected by the light receiving means andcombining the contrast waveforms.
 13. A defect inspector apparatus asset forth in claim 12, further comprising a defect detecting means fordetecting a defect of a pattern based on the contrast waveforms.
 14. Adefect inspection apparatus as set forth in claim 12, wherein theinspected surface comprises the surface of a film stacked on asemiconductor substrate and formed into a predetermined pattern.
 15. Adefect inspection apparatus as set forth in claim 14, wherein: thepatterns are formed symmetrical about a predetermined center line andthe defect detecting means detects a part which is not symmetrical aboutthe center line in the contrast waveform data obtained by the contrastwaveform generating means as a defect.
 16. A defect inspection apparatusfor inspecting for a defect present in an inspected surface, comprising:a light source for emitting a beam of light of a predetermined frequencyband; a light frequency changing means for receiving as its input thebeam of light emitted from the light source and outputting the relatedbeam of light converted to a plurality of beams of inspection light anda beam of reference light beam having close frequencies different fromeach other; a light focusing means for focusing the related beams ofinspection light output from the light frequency changing means on theinspected surface to form focal points; a scanning means for scanningthe focused beams of inspection light on the inspected surface; asuperposing means for superposing the beam of reflected light of thebeams of inspection light from the inspected surface and the beam ofreference light on each other to cause interference between the beams ofreflected light and the beam of reference light; a confocal defectingmeans upon which the superposed light of the beams of reflected lightand the beam of reference light is incident and detecting the intensityof the superposed light by confocal detection; and a contrast waveformgenerating means for generating contrast waveforms reflecting a surfaceshape of the inspected surface in the scanning direction at the focalpositions based on the light intensity detected by the control defectingmeans.
 17. A defect inspection apparatus as set forth in claim 16,further comprising a defect detecting means for detecting defects of thepattern based on the contrast waveforms.
 18. A defect inspectionapparatus as set forth in claim 16, wherein the inspected surfacecomprises the surface of a film stacked on a semiconductor substrate andformed into a predetermined pattern.
 19. A defect inspection apparatusas set forth in claim 18, wherein: the patterns are formed symmetricalabout a predetermined center line and the defect detecting means detectsa part which is not symmetrical about the center line in the contrastwaveform data obtained by the contrast waveform generating means as adefect.
 20. A defect inspection apparatus as set forth in claim 15,wherein said confocal defecting means has: an aperture plate having asmall aperture for passing the superposed light and a light receivingelement for detecting the intensity of the superposed light passed saidaperture.